Nondestructive measurement of a film thickness is an important objective for many industries. For example, in the aircraft industry, paint for functional and decorative purposes is often applied to aircraft surfaces, which may have near critical weight. Also, prior to applying a surface paint, care must be taken to ensure that an adequate amount of primer is uniformly applied with an adequate thickness to act as a proper corrosion inhibitor. For example, in some aircraft applications, a paint film thickness of 0.4 to 1.0 thousandth of an inch (mil) over the surface of the substrate is desirable. Moreover, depending on the model of aircraft, over 500 pounds of primer and topcoat may be applied to an aircraft in accordance with customer specifications.
Since the weight and distribution of paint on aircraft surfaces may sometimes affect the performance of the aircraft it is desirable to apply and measure the paint in a controlled means in order to generally apply a minimum adequate thickness. This measurement is desirable to ensure that the primer is sufficiently thick for corrosion protection, but not so thick as to unnecessarily add to the weight of the airplane, which can negatively affect fuel efficiency. After the primer thickness is determined to be of sufficient thickness, a top coat of paint can be applied. If the top coat is too thin, it is a cosmetic problem. If it is too thick, it will again add to the weight and affect performance.
For very thick coating of paint on a composite material there is an additional problem. Very thick coatings on a composite surface can provide an electrical path for lightning strike. The lightning will track along the thick paint rather than go directly through the paint to ground.
Aircraft components that are made of a metallic material can have their coatings measured by several conventional commercial tools. However, components are increasingly manufactured from carbon or glass fiber reinforced composite materials. These materials are difficult to measure using conventional techniques since they have relatively low direct-current conductivity and magnetic permeability.
Another conventional alternative is to wait for the paint to dry and indirectly measure the paint film thickness by weighing the component in question. This can result in an undesirable cycle delay of up to three days. For example, when primer needs to be measured on a composite component, the component may first be weighed, then painted and finally reweighed. In addition, this process can be prohibitively difficult in the case of large components, for example, an airplane fuselage. Since an acceptably accurate and dependable direct measurement technology for composite materials is not available, when the primer film is suspected of being excessively thick or uneven, the component typically must be disassembled and repainted at significant cost and risk of damage.
Commercial ultrasonic measurement systems, such as the Panametric 45L Deluxe® and/or the pulse echo linear transducer (PELT) produced by Defelsko, are available. These commercial products will give some paint measurement thickness data for some types of composites. The ultrasonic systems are useable for relatively thick coatings, but for thin coatings of paint the ultrasonic system is not sufficiently reliable and accurate to meet some specification requirements. Another concern is the additional training required needed to perform the ultrasonic thickness measurement as the method requires a practiced eye to discern the correct features of the returned signal to make a valid measurement.
Additionally, the nondestructive measurement of film thickness is of concern to other industries, such as the automotive industry and the paper production industry. In the automotive industry, the analysis of primer film thickness is important, for example, to determine whether additional touchup work is required for some of the paint applications common to the industry. In the paper industry, verifying the uniformity of the paper thickness can be an important quality control issue. With the increasing use of composite materials, it has become apparent that current methods of measurement that perform adequately with metallic substrates or with thick films, but not with substrate materials that have relatively low direct-current conductivity and magnetic permeability or with thin films, are not sufficient.
The foregoing discussion derives primarily from U.S. Pat. No. 7,173,435, which discloses a thickness measuring apparatus and method using a microwave cavity resonator. Referring to FIG. 1, which corresponds to FIG. 1 of U.S. Pat. No. 7,173,435, a measurement device generally comprises a resonant cavity 18, a radio frequency (RF) input 24, and a signal detector 34. The resonant cavity has a chamber 20 with an open flat end 22 which is placed over a sample 12 having a substrate 16 and a dielectric layer 14. The RF input is connected to the chamber using a connection 26 which may be, for example, a coaxial cable. The signal detector 34 is also connected via its own connection to the chamber by a connection 32, which may be, for example, a coaxial cable. In addition, a processor 36 is provided to correlate the output power of the resonant cavity to a thickness of the dielectric layer. With this device, thickness of the dielectric is determined by accurately calibrating to known thickness samples.
The essential element of this and any thickness measurement instrument is the sensor, in this case, the resonant RF cavity. Previous patents including the one mentioned above do not provide the design details of this essential element. It has been found that without the correct design of the resonant cavity, the sensor, the measurement instrument functions very poorly. This present disclosure establishes the basis for thickness measurement using a frequency in the microwave range and details the design of this essential element.